Multilayered, bioriented, heat shrinkable film, method for the manufacture thereof and use thereof for packaging food products and consumer goods

ABSTRACT

A multilayer, biaxially oriented, heat shrinkable thermoplastic film includes a layer having between 50 and 100 parts by weight ethylene/alpha-olefin C 4  -C 12  copolymer, and between 0 and 50 parts by weight of a polymer selected from polyolefin, and modified polyolefin, the layer having more than one melting point when detected by diffential scanning calorimetry according to ASTM D-3418; wherein the total thickness variation in the multilayer film is lower than ±10%, and wherein the average deviation of the multilayer film from a straight line is lower than ±40 mm, and wherein the sag average of the multilayer film is lower than 25 mm. A method of making the film, and a package made with the film, are also disclosed.

FIELD OF THE INVENTION

The present invention relates to a multilayer, bioriented, heatshrinkable film, a method for the manufacture thereof and use thereoffor packaging food products and consumer articles.

More particularly, the present invention relates to a multilayer,bioriented, heat shrinkable film extruded from a flat die and stretchedboth in machine direction and in cross-direction wherein at least onelayer comprises at least one ethylene/alpha-olefin C₄ -C₁₂ copolymer.

BACKGROUND OF THE INVENTION

Multilayer heat shrinkable films have been known for a long time.

U.S. Pat. No. 4,532,189 (W.R. Grace & Co.) discloses a multilayer, heatshrinkable film comprising:

(A) a core (central) layer comprising a linear low density polyethyleneor a linear medium density polyethylene;

(B) two skin (external) layers comprising a blend of from 70% to 90%, byweight, of an ethylene/propylene copolymer and from 10% to 30%, byweight, of a propylene homopolymer;

wherein said film has an average machine direction free shrink at 200°F. of at least 12% and an average cross-direction free shrink at 200° F.of at least 17%.

The core layer may also comprise other polymers such as, for example,ethylene/propylene copolymers, ethylene/vinyl acetate copolymers,ionomer resins and non-linear low density poly ethylenes.

Moreover, said film may also comprise two intermediate layers comprisinga blend of approximately 90% by weight of an ethylene/vinyl acetatecopolymer and approximately 10% of an ionomer resin.

U.S. Pat. No. 4,551,380 (W.R. Grace & Co.) discloses a multilayer heatshrinkable film comprising:

(A) a cross-linked core layer consisting essentially of a linear lowdensity polyethylene; and

(B) two surface layers comprising essentially a blend of (1) a linearlow density polyethylene, (2) a linear medium density polyethylene and(3) an ethylene/vinyl acetate copolymer.

GB-A-2,097,324 discloses heat shrinkable films manufactured bystretching, at least three times their original linear dimension in atleast one direction, a film having the following homogeneouscomposition:

(A) 5-100%, by weight, of at least one linear copolymer of ethylene withat least one C₈ -C₁₈ alpha-olefin, said copolymer having the followingcharacteristics:

(a) melt index of 0.1-4.0 g/10 min.;

(b) density of 0.900-0.940 g/cm³ ;

(c) stress exponent above 1.3; and

(d) two distinct crystallite melting regions below 128° C. as determinedby differential scanning calorimetry (DSC), the temperature differencebetween said regions being at least 15° C.; and

(B) 0-95%, by weight, of at least one polymer selected from the groupconsisting of ethylene homopolymers and copolymers of ethylene with anethylenically unsaturated comonomers, said polymer having only onecrystallite melting point below 128° C.;

with the proviso that stretching is carried out within the temperaturerange defined by the two melting points of the crystallites of theethylene/alpha-olefin copolymer of the above paragraph (A).

These films are manufactured by the well-known air bubble technique. Anexample of method and equipment of this technique is disclosed by U.S.Pat. No. 4,841,605.

However, the films obtained with this technique have the disadvantage ofnot having sufficiently uniform thickness and planarity. In fact thetotal thickness variation in said films is ±15% while, as regardsplanarity, it has defects consisting of deviations from a straight line(snaking) and sags. More particularly the average deviations from astraight line (snaking) is approximately 50 mm, whereas the average sagis approximately 35 mm.

In addition to the air bubble technique, stretching the films also bythe so-called "tenter frame" technique is known. Examples of machinessuitable for implementing this technique are disclosed by U.S. Pat. Nos.3,148,409 and 3,201,826.

The tenter frame technique, also known as "flat orientation technology",consists of extruding a film-forming material through a flat die over achill roll, preferably immersed in water, to chill the molten film.

In the case of multilayer films the various polymers or blends ofpolymers are generally coextruded by conventional techniques but, whenonly a few of the layers have to undergo special treatments, such as forexample irradiation with fast electrons to induce cross-linking, onlythe layer or layers to be treated is extruded or are coextruded, thetape obtained in this way is subjected to the required treatment andthen the remaining layers are extruded on the same.

The tape is then oriented, by stretching, in two separate and successivesteps, although devices able to stretch the tape simultaneously in bothdirections are known (U.S. Pat. No. 3,148,409).

Generally stretching is performed first in machine direction (MD) andthen in cross-direction (TD).

MD stretching is usually carried out by passing the tape through pairsof rolls which rotate at different speeds. At least one of the firstpairs of rolls is heated, for example by inner circulation of hot oil.

TD stretching is usually performed in a tenter frame oven whichcomprises a certain number of heating zones and suitable stretchingmeans.

Typically a tenter frame oven comprises from three to six zones: one totwo for preheating the tape, one to two for stretching it in thecross-direction and one to two for relaxing and winding the film. Eachzone may be heated at a different temperature level.

This technique has not however been adopted in the manufacture of heatshrinkable films based on ethylene copolymers.

Only EP-A-405 916 discloses the utilization of this technique in themanufacture of an extruded, bioriented, mono or multilayer film, whereinthe film-forming polymers of at least one layer consist of:

(A) 75-100%, by weight, of at least one linear ethylene/alpha-olefincopolymer having a density of between 0.890 g/cm³ and 0.930 g/cm³, and

(B) 25-0%, by weight, of a linear high density polyethylene having adensity of between 0.935 and 0.960 g/cm³, with the proviso that thetotal of the film-forming polymer (A) and (B) has a single melting pointas determined by differential scanning calorimetry according to ASTMD-3417.

Moreover, EP-A-405 916, page 3, lines 18-21, reports that attempts madeto apply the tenter frame technique to the polymers of GB-A-2 097 324have not given satisfactory results because the films obtained in thisway were highly sensitive to minimal variations of the processparameters, such as stretching temperature, stretching ratio and thespeed of the manufacturing line.

Although EP-A-405 916 also refers to multilayer films, its examples onlyrefer to monolayer films.

On the other hand, multilayer films have, compared to monolayer ones,the considerable advantage of allowing to combine one with the otherseveral layers having different physical and chemical properties in viewof the required properties of the final film.

Therefore the need for heat shrinkable multilayer films, extruded from aflat die, bioriented by the tenter frame technique, wherein at least onelayer comprises ethylene copolymers, is still greatly felt.

In fact, the films manufactured by this technique have severaladvantages over those manufactured by the air bubble technique.

A first advantage consists of the fact that the stretching ratios inmachine direction and cross-direction may vary as required whereas inthe air bubble technique they are always substantially equal one to theother.

A second advantage is that the stretching ratio may be preselectedwithin a relatively wide range, typically of betweeen 2:1 and 12:1,while in the air bubble technique it must be between 3:1 and 6:1.

A third advantage is that the sealing agents can be selected as requiredwhereas in the air bubble technique they have to be selected in therestricted range of sealants whose softening point is not substantiallylower than the stretching temperature to prevent the sealant fromsoftening during the heating step, with consequent sealing of theopposite walls of the bubble.

A further advantage consists of the fact that the thickness andplanarity of the films manufactured by this technique are much moreuniform than in those manufactured with the air bubble technique.

An additional advantage is that, being able to control the thicknessbetter, thinner films can be manufactured, consequently saving materialsand reducing environmental impact.

OBJECTS OF THE INVENTION

In accordance with the above, a first object of the present invention isto provide a multilayer heat shrinkable film, extruded from a flat dieand bioriented by the tenter frame technique, wherein at least one layerconsists of at least 50%, by weight, of an ethylene/alpha-olefin C₄ -C₁₂copolymer or of a blend of ethylene/alpha-olefin C₄ -C₁₂ copolymers.

A second object of this invention is to provide a multilayer heatshrinkable film wherein the total variation in thickness is lower than±10% and, preferably, than ±5%.

A third object of this invention is to provide a multilayer heatshrinkable film wherein the average of the deviations from a straightline (snaking) is lower than 40 and, preferably, than 25 mm, and averageof the sags is lower than 25 mm.

A fourth object of this invention is to provide a multilayer heatshrinkable film which has been stretched with a stretching ratio of from2:1 to 11:1 in machine direction and from 3:1 to 12:1 in cross-directionand, preferably, with a stretching ratio of from 4.5:1 to 9.5:1 inmachine direction and from 5:1 to 11:1 in cross-direction.

Another object of this invention is to provide a method for themanufacture of a multilayer heat shrinkable film having the abovecharacteristics.

A further object of this invention is to provide the use of a multilayerheat shrinkable film having the above characteristics to package foodproducts and consumer articles.

These and other objects have been achieved by the film describedhereinbelow.

DEFINITIONS

Unless otherwise stated, in this description and in the claims, thefollowing symbols have the meanings set forth hereinbelow.

A. Extrusion

S_(e) =linear speed (m/min.) of the quenched cast flat tape

T_(c) =chill roll temperature.

B. Longitudinal orientation

L_(r) =longitudinal stretching ratio

T_(p) =rolls temperature (°C.) before longitudinal orientation(preheating)

T_(l) =rolls temperature (°C.) during longitudinal orientation

MD=machine or longitudinal direction

C. Cross-direction orientation

C_(r) =cross-direction stretching ratio

T_(cp) =preheating temperature (°C.) in the cross-direction

T_(cs) =stretching temperature (°C.) in the cross-direction

T_(cr) =relaxation temperature (°C.) in the direction

S_(c) =winding speed

TD=cross-direction

D. Properties of the films

ST=shrink tension

This property is measured by gradually heating the film and measuringthe maximum force developed per section unit of the film itself.

S %=shrink percentage

This property is measured by heating the film at 120° C. for at least 5sec., and then measuring the percentage of shrinkage in the longitudinaldirection and in the cross-direction.

H=haze

This property is measured according to ASTM D 1003-61.

G=Gloss

This property is measured according to ASTM D 2457-90.

M=elastic modulus

This property is measured according to ASTM D 882-90.

TS=tensile strength

This property is measured according to ASTM D 882-90.

E=elongation

This property is measured according to ASTM D 882-90.

TTV=total thickness variation

This property is measured according to ASTM D 374-88.

PTV=planarity variation

The main planarity defects consist of bag & sag and snaking.

The device used to measure these defects essentially consists of

1. an unwind section with locking devices,

2. a dancing rolls to allow proper tension adjustment,

3. two idle rolls spaced circa 4 meters one from the other to supportthe film span under measurement,

4. a reference scale for proper film alignment close to each idle roll,

5. a pair of rubber clamps actuated by a pedal lever to block the filmside opposite to the unwind section,

6. a motorised rewind section,

7. a device for measuring the length of the rewound film between twosets of measurement,

8. a guide, positioned substantially at the center between the two idlerolls, which supports:

a) a feeler fixed to a sliding support unit provided with a scale formeasuring the horizontal distance covered by the feeler from a prefixedzero point corresponding to the left edge of the film and a second scalefor measuring vertical movements. The zero on this scale has been fixedby taking into consideration the natural drop midway in a perfectly flatfilm span,

b) a slide fixed at the zero reference point,

9. various couples of weights to be attached to the dancing roller toadjust the tension of the film according to the type, width andthickness of the film itself.

In order to perform the measurements, a roll of film is placed in theunwind section and a length of film is unwound.

The unwind station is locked.

The film is aligned and fixed to the wind-up core.

The rewind section is locked.

Using the vertical scale, the position of the flattest part of the filmis measured.

The bag and sag in the film are then measured by recording thedifference between the film levels at each point and the level of theflattest part of the film.

Measurements are taken along the whole film width from the left to theright side so as to obtain a representative profile of the surface. Forthis purpose measurements every 5-10 cm are recommended.

The data are reported on paper, i.e. sag and relevant position (left,center or right).

The whole operation is performed twice to four times to measure from twoto four lengths of film.

Snaking, also referred to as "banana effect", represents the deviationof film from a straight line for a given length of material.

The measurements are performed on a length of film of 4 meters using thedevice and method described above.

Cross-linking is measured according to ASTM D 2765-90.

The term Mrad denotes the dose absorbed by the tape or by the filmduring treatment of irradiation with high energy electrons, and isequivalent to 10⁴ J/Kg.

Moreover, unless otherwise stated in this description and in the claims,the following terms have the meanings indicated hereinbelow.

The term "film" means a flat and flexible material having a thickness ofbetween 5 and 150 microns.

The term "heat shrinkable film" means a bioriented film which shrinks byat least 15% at 120° C. in both directions.

The term "tape" means a layer of material before orientation and hastypically a thickness of between 180 microns and 3 mm.

The term "polyolefin" means thermoplastic resins obtained byhomopolymerisation or copolymerisation of olefins with other monomers,wherein the olefin units are present in larger amounts than othermonomers. The term "polyolefin" comprises but is not limited to"ethylene/alpha-olefin copolymer", "ethylene/vinyl acetate (EVA)copolymer", "ethylene/acrylate copolymer", "propylene copolymer" and"polypropylene" (PP) as defined hereinafter.

The term "modified polyolefins" means polyolefins characterised by thepresence of functional groups, such as anhydride and/or carboxyl acidgroups.

The term "ethylene/alpha-olefin copolymer" means a copolymer of ethyleneand one or more C₄ -C₁₂ alpha-olefins and is preferably selected fromthe group comprising linear copolymers of ethylene and butene-1,4-methyl-pentene-1, hexene-1 or octene-1.

The term "ethylene/vinyl acetate (EVA) copolymer" means anethylene/vinyl acetate copolymer wherein the ethylene units are presentin larger amounts than the vinyl acetate units.

The term "ethylene/acrylate copolymer" means a compound obtained bycopolymerisation, according to known techniques, of ethylene withacrylic monomers of the formula ##STR1## where R is H or C₁ -C₄ alkyl,and

X is H, C₁ -C₄ alkyl or a metal cation, preferably chosen

from Zn++ and Na+,

wherein the ethylene units are present in larger amounts than theacrylic units.

The term "propylene copolymer" means a copolymer of propylene andethylene and/or butene-1 wherein the propylene units are present inlarger amounts than the ethylene or butene units.

The term "polypropylene" (PP) means a thermoplastic resin obtained byhomopolymerisation of propylene according to conventional techniques.

SUMMARY OF THE INVENTION

It has now been found that the abovementioned objects are achieved witha multilayer, bioriented, heat shrinkable film extruded from a flat diewherein at least one layer comprises an ethylenelalpha-olefin C₄ -C₁₂copolymer, characterised in that said layer comprises

A₁) 50-100 parts, by weight, of an ethylene/alpha-olefin C₄ -C₁₂copolymer or a blend of ethylene/alpha-olefin C₄ -C₁₂ copolymers, and

A₂) 50-0 parts, by weight, of a polymer selected from polyolefins,modified polyolefins and blends thereof,

with the proviso that this layer has more than one melting point asdetermined by differential scanning calorimetry according to ASTMD-3418.

The present invention relates therefore to a multilayer, bioriented,heat shrinkable film extruded from a flat die wherein at least one layercomprises an ethylene/alpha-olefin C₄ -C₁₂ copolymer, characterised inthat:

(a) said layer comprises

A₁) 50-100 parts by weight of an ethylene/alpha-olefin C₄ -C₁₂ copolymeror a blend of ethylene/alpha-olefin C₄ -C₁₂ copolymers, and

A₂) 50-0 parts by weight of a polymer chosen from among polyolefins,modified polyolefins and blends thereof,

with the proviso that this layer has more than one melting point asdetermined by differential scanning calorimetry according to ASTMD-3418,

(b) the total variation in thickness of said film is lower than ±10%,and

(c) the average deviation from a straight line (snaking) of said film isless than 40 mm.

(d) the average sag is smaller than 25 mm.

The polyolefins are preferably selected from ethylene/vinyl acetatecopolymers, ethylene/acrylate copolymers, propylene copolymers andpolypropylenes.

The melting behavour of ethylene/alpha-olefin C₄ -C₁₂ copolymers dependson many factors related to the copolymerisation process: quantity of thealpha-olefin, nature of the catalyst, comonomer feed rate, temperatureand other process conditions which may have a significant effect on thedistribution of the alpha-olefin in the copolymer chains.

Examples of suitable alpha-olefins are butene-1, 4-methylpentene-1,hexene-1 and octene-1.

According to the present invention, the content of alpha-olefin in theethylene/alpha-olefin C₄ -C₁₂ copolymer is preferably of at least 7%, byweight, and the alpha-olefin is octene-1.

Even more preferably the ethylene/alpha-olefin C₄ -C₁₂ copolymer is alinear ethylene/octene-1 copolymer which has a octene-1 content of 10%±2%, by weight, a density of circa 0.920 g/cm³ and three melting pointsat 106 ±2; 119 ±2 and 122 ±2° C.

Advantageously the layers comprising an ethylene/alpha-olefin C₄ -C₁₂copolymer are the core layer of a film having three layers or the skinlayers of a film having five layers.

The two skin layers of a three-layer film and the intermediate layers ofa five-layer film may have a composition which is different one from theother. Nevertheless, they preferably have the same composition andessentially consist of one or more film-forming polymers chosen fromamong ethylene/vinyl acetate copolymers, ethylene/acrylate copolymers,polypropylene, polypropylene copolymers, modified polyolefins and blendsthereof.

The method for manufacturing a multilayer, bioriented, heat shrinkablefilm according to the present invention comprises, preferably,stretching of the film, at appropriate temperatures, first in machinedirection and then in cross-direction.

The use of multilayer, bioriented, heat shrinkable film according to thepresent invention for packaging articles is of the conventional type:the film is wound around the article, sealed or clipped and shrunk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a first preferred embodiment according tothe present invention consisting of a three-layer bioriented film.

FIG. 2 is a cross section a second preferred embodiment according to thepresent invention consisting of a five-layer bioriented film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, which is a cross section of a preferred bioriented,three-layer film according to the present invention, it can be seen thatthis embodiment comprises a core layer 2 and two skin layers 1 and 3.

Preferably, the thickness of the film is of from 8 to 25 microns. Thepreferred thickness of the core or internal layer is in the range offrom 1 to 23 microns whereas that of each of the skin (external) layersis in the range of from 1 to 12 microns.

The core layer comprises

A₁) 50-100 parts, by weight, of an ethylene/alpha-olefin C₄ -C₁₂copolymer or a blend of ethylene/alpha-olefin C₄ -C₁₂ copolymers, and

A₂) 50-0 parts, by weight, of a copolymer chosen from amongethylene/vinyl acetate copolymers, ethylene/acrylate copolymers,polypropylenes, polypropylene copolymers, modified polyolefins andblends thereof,

with the proviso that this layer has more than one melting point asdetermined by differential scanning calorimetry according to ASTMD-3418.

Typically, the core layer comprises 100%, by weight, of anethylene/alpha-olefin C₄ -C₁₂ copolymer or a blend ofethylene/alpha-olefin C₄ -C₁₂ copolymers which have more than onemelting point as determined by differential scanning calorimetryaccording to ASTM D-3418.

A typical example of film-forming composition suitable for forming thelayer 2 of FIG. 1 consists of 100% of an ethylene/octene-1 copolymerwhich contains 10% of octene-1 and as a density of 0.920 g/cm³ and threemelting points at 106 ±2; 119 ±2 and 122 ±2° C.

When manufacturing a first preferred embodiment of FIG. 1, it mayhowever be advantageous to recycle the remains of film-formingcomposition and scraps, of tape and/or film which are generated duringthe various steps of the manufacturing process, adding them to thefilm-forming composition of the core layer. In this case, thefilm-forming composition of the core layer will contain up to 25%, byweight, of a polymer chosen from polyolefins, modified polyolefins andblends thereof.

Preferably, both the skin layers 1 and 3 of FIG. 1 have the samecomposition and essentially consist of a polymer chosen fromethylene/vinyl acetate copolymers, ethylene/acrylate copolymers,polypropylenes, polypropylene copolymers, modified polyolefins andblends thereof.

Examples of preferred polyolefins for these layers areethylene/alpha-olefin C₄ -C₁₂ copolymers, propylene copolymers andpolypropylenes.

A first typical example of a suitable film-forming composition for bothlayers 1 and 3 of FIG. 1 is the following:

40-60%, by weight, of a first ethylene/alpha-olefin C₄ -C₁₂ copolymer,

20-30%, by weight, of a second ethylene/alpha-olefin C₄ -C₁₂ copolymer,

20-30% by weight of an ethylene/vinyl acetate copolymer (EVA).

A second typical example of film-forming composition suitable for bothlayers 1 and 3 of FIG. 1 is the following:

75-100%, by weight, of a copolymer of propylene,

0-25%, by weight, of a polypropylene (PP).

The second preferred embodiment shown in FIG. 2 relates to a five-plybarrier film of this invention comprising a core layer 2, two tie (orintermediate) layers 4 and 5, and two skin layers 1 and 3.

Preferably, the thickness of this film is of from 15 to 45 microns.

The core layer 2 of the embodiment shown in FIG. 2 acts as a barrieragainst oxygen permeation and is made of any of the known barrierpolymeric resins, such as ethylene/vinyl alcohol copolymers (EVOH) andvinylidene chloride copolimers (PVDC). The term EVOH refers tosaponified products of ethylene/vinyl ester copolymers, generally ofethylene-vinyl acetate copolymers, wherein the ethylene content istypically comprised between 20 and 60% by mole and the degree ofsaponification is generally comprised between 85 and 99.5%. The EVOH canbe employed as such or blended with other EVOHs or with one or morepolyamides. The term PVDC refers to a vinylidene chloride copolymerwherein a major amount of the copolymer comprises vynilidene chlorideand a minor amount of the copolymer comprises one or more unsaturatedmonomers copolymerizable therewith, typically vinyl chloride, and alkylacrilates (e.g. metyl acrylate) or to a blend thereof in differentproportions. Generally said PVDC will contain plasticisers and/orstabilizers as known in the art.

The composition of the two tie layers 4 and 5 of the embodiment shown inFIG. 2 may be the same or different. Advantageously the tie layers 4 and5 have the same composition and are made of a modified polyolefin.

A typical example of film-forming composition suitable for forming thelayers 4 and 5 of FIG. 2 consists of 100% of a maleic anhydride graftedethylene/butene copolymer containing 6% by weight of butene.

Preferably both skin layers 1 and 3 of the embodiment shown in FIG. 2have the same composition and comprise

A₁) 50-100 parts, by weight, of an ethylene/alpha-olefin C₄ -C₁₂copolymer or a blend of ethylene/alpha-olefin C₄ -C₁₂ copolymers, and

A₂) 50-0 parts, by weight, of a copolymer chosen from among polyolefins,modified polyolefins and blends thereof,

with the proviso that at least one of these layers has more than onemelting point as determined by differential scanning calorimetryaccording to ASTM D-3418.

Typically, the skin layers 1 and 3 of the second preferred embodiment ofFIG. 2 comprise 100%, by weight, of an ethylene/alpha-olefin C₄ -C₁₂copolymer or a blend of ethylene/alpha-olefin C₄ -C₁₂ copolymers whichhave more than one melting point as determined by differential scanningcalorimetry according to ASTM D-3418.

A typical example of a film-forming composition suitable for forming thelayers 1 and 3 of FIG. 2 consists of 100% of an ethylene/octene-1copolymer which contains 10% of octene-1 and has a density of 0.920g/cm³ and three melting points at 106 ±2; 119 ±2 and 122 ±2° C.

When manufacturing a second preferred embodiment according to FIG. 2, itmay be advantageous to recycle the remains of film-forming compositionand scraps, of tape and/or film, which are generated during the varioussteps of the manufacturing process, adding them to the film-formingcomposition for the skin layers 1 and 3 or, preferably, for theintermediate layers 4 and 5. Alternatively, and preferably in this case,additional inner layer(s) can be formed composed exclusively of recycledfilm. In any case the film will contain up to from 30% to 40% by weight,preferably up to 25%, even most preferably from about 7% to about 20% ofrecycled material.

A first specific example of composition suitable for manufacturing amultilayer, bioriented, heat shrinkable film according to the presentinvention is the following:

Film-forming composition of the core layer 2 (FIG. 1):

100% of an ethylene/octene-1 copolymer which contains 10% octene-1 (NMRdetermination) and has a density of 0.920 g/cm³ and three melting pointsat 106 ±2; 119 ±2 and 122 ±2° C.

Film-forming composition of the skin layers 1 and 3 (FIG. 1):

50.1%, by weight, of an ethylene/octene-1 copolymer which contains 10%of octene-1 (NMR determination) and has a density of 0.920 g/cm³ andthree melting points at 106 ±2; 119 ±2 and 122 ±2° C.;

24.8%, by weight, of an ethylene/octene-1 copolymer which contains 3.8%of octene-1 (NMR determination) and has a density of 0.926 g/cm³ and asingle melting point at 122.5 ±2° C.;

24.6%, by weight, of an ethylene/vinyl acetate copolymer which contains4.5% of vinyl acetate and has a density of 0.926 g/cm³ and a singlemelting point at 106.8 ±2° C.;

0.5% of slip and anti-block agents.

It will be noted that not only the core layer but also the skin layershave more than one melting point and come within the scope of thepresent invention also as regards their composition.

A second specific example is the following:

Film-forming composition of the core layer 2 (FIG. 1):

100% of an ethylene/octene-1 copolymer which contains 10% of octene-1(NMR determination) and has a density of 0.920 g/cm³ and three meltingpoints at 106 ±2; 119 ±2 and 122 ±2° C.

Film-forming composition of the skin layers 1 and 3 (FIG. 1):

87.5%, by weight, of a propylene/ethylene copolymer which contains 3.5%of ethylene and has a density of 0.900 g/cm³ and a single melting pointat 135.9° C.;

12.1%, by weight, of polypropylene having a density of 0.904 g/cm³ and asingle melting point at 160° C.;

0.4% of slip and anti-block agents.

A third specific example of composition suitable for manufacturing amultilayer, bioriented, heat shrinkable film according to the presentinvention is the following:

Film-forming composition of the core layer 2 (FIG. 2):

80% by weight of an etylene/vinyl alcohol copolymer containing 44%ethylene,

20% by weight of a caprolactam/laurolactam copolymer melting at 128-132°C.;

Film forming composition of the intermediate layers 4 and 5 (FIG. 2):

100% by weight of a maleic anhydride grafted ethylene/butene copolymercontaining 6% by weight of butene;

Film-forming composition of the skin layers 1 and 3 (FIG. 2):

50.1%, by weight, of an ethylene/octene-1 copolymer which contains 10%of octene-1 (NMR determination) and has a density of 0.920 g/cm³ andthree melting points at 106 ±2; 119 ±2 and 122 ±2° C.;

24.8%, by weight, of an ethylene/octene-1 copolymer which contains 3.8%of octene-1 (NMR determination) and has a density of 0.926 g/cm³ and asingle melting point at 122.5 ±2° C.;

24.6%, by weight, of an ethylene/vinyl acetate copolymer which contains4.5% of vinyl acetate and has a density of 0.926 g/cm³ and a singlemelting point at 106.8 ±2° C.;

0.5% of slip and anti-block agents.

It will be noted that the skin layers 1 and 3 have more than one meltingpoint and come within the scope of the present invention.

Examples of suitable slip agents are erucamide (available from HumkoChemical under the trademark Kemamide E). Other examples of suitableslip agents are stearamide (available from Humko Chemical under thetrademark Kemamide S) and N,N'-dioleoylethylenediamine (available fromGlyco Chemical under the trademark Acrawax C).

An example of a suitable anti-block agent is represented by diatomaceoussilica (available from McCullough & Benton, Inc. under the trademarkSuperfine Superfloss). Other suitable anti-block agents are syntheticsilicas such as those manufactured and sold by the Davison Division ofWR Grace, under the trademark Syloid.

As is well-known to those skilled in the art, other suitable additivesfor film-forming compositions are antioxidants, heat stabilisers,organic or inorganic coloured pigments, anti-fog agents, anti-staticagents, UV absorbers and the like.

These types of additives may be added to the film-forming compositionsof the present invention according to criteria which are well-known tothose skilled in the art, without however departing from the scope ofthe present invention.

In a typical embodiment of the present invention, the extruded andcooled tape is irradiated, before stretching, with high energyelectrons. Said irradiation may however also be performed during orafter stretching.

As is well-known to those skilled in the art, this type of irradiationcauses cross-linking of the polymers and improves their mechanicalproperties, especially at high temperatures.

The preferred level of radiation is of from 1 to 12 Mrad.

Even more preferably the level of radiation is of from approximately 1.5to approximately 5 Mrad.

The preferred conditions for orienting the multilayer films of thepresent invention comprise first of all stretching in machine directionat a temperature of from 60 to 150° C., preferably of from 80 to 120°C., and then stretching in cross-direction, preheating the film to atemperature of from 80 to 190° C., preferably of from 110 to 170° C.,stretching it at a temperature of from 60 to 160° C., preferably of from80 to 140° C., and, finally, relaxing it at a temperature of from 50 to150° C., preferably of from 75 to 130° C.

Generally, the stretching ratios according to the invention range from2:1 to 11:1 in machine direction and from 3:1 to 12:1 in cross-directionand, preferably, from 4.5:1 to 9.5:1 in machine direction and from 5:1to 11:1 in cross-direction.

Unlike multilayer, bioriented, heat shrinkable films manufactured withthe same composition but by the air bubble technique, the multilayer,bioriented, heat shrinkable film of the present invention ischaracterised by a total thickness variation lower than ±10%. Typicallysaid total variation in thickness is lower than ±5%.

Moreover, the film according to the present invention is characterisedin that the average deviation from a straight line (snaking) is smallerthan 40 mm and that the average sag is smaller than 25 mm.

Typically, the average deviation from a straight line (snaking) issmaller than 25 mm.

The preferred use of the multilayer, bioriented, heat shrinkable filmaccording to the present invention consists of packaging articles ofvarious types. Preferably food products and consumer articles.

In practice, the film is wound around the article, sealed or clipped andshrunk according to conventional techniques. For example, by exposingthe article wound in the film to a current of hot air or hot water.

The following examples are intended to illustrate the present inventionwithout however limiting it in any way. The melting points set forth inthe description, in the claims and in the following examples have beenmeasured by differential scanning calorimetry according to ASTM D-3418.

EXAMPLES 1-5

Film-forming composition of the core layer 2 (FIG. 1):

100% of an ethylene/octene-1 copolymer which contains 10% of octene-1(NMR determination) and has a density of 0.920 g/cm³ and three meltingpoints at 106 ±2; 119 ±2 and 122 ±2° C.

Film-forming composition of the skin layers 1 and 3 (FIG. 1):

50.1%, by weight, of an ethylene/octene-1 copolymer which contains 10%of octene-1 (NMR determination) and has a density of 0.920 g/cm³ andthree melting points at 106 ±2; 119 ±2 and 122 ±2° C.;

24.8%, by weight, of an ethylene/octene-1 copolymer which contains 3.8%of octene-1 (NMR determination) and has a density of 0.935 g/cm³ and asingle melting point at 122.5 ±2° C.;

24.6%, by weight, of an ethylene/vinyl acetate copolymer which contains4.5% of vinyl acetate and has a density of 0.926 g/cm³ and a singlemelting point at 106.8 ±2° C.;

0.5% of slip and anti-block agents.

Note: the aforesaid blend of layers 1 and 3 has two melting points at102.1 and 120.5 ±2° C.

Production was performed on Brueckner lines.

The film-forming compositions were coextruded through a flat die and thecast tape was chilled on a chill roll partially immersed in a water bathat a temperature of from 5 to 60° C. (T_(c)) with the aid of an airknife. The linear speed of the quenched tape was of from 6 to 8 m/min(S_(e)).

The tape was then heated to a temperature of from 110 to 115° C. on oilheated rolls stretching the tape in machine direction with ratios offrom 6:1 to 9.5:1.

The tape was then transferred to a tenter frame oven having four heatingzones in which the temperature was of from 115 to 130° C. and then, inexamples 1, 3, 4, 5, in a stretching zone heated to temperatures of from115° C. to 127° C., with stretching ratios of from 5.8:1 to 6.1:1 andfinally in a relaxation zone heated to a temperature of from 80 to 85°C.

In example 2, 2 stretching zones and no relaxation zone were used.

Finally the film was cooled and wound onto a roll at a speed of from 30to 50 m/min. (S_(c)).

The multilayer bioriented films obtained in this way have a thickness of15 microns.

The ratio between layers 1, 2 and 3 was 1:2:1, respectively.

The process conditions (PC) of Examples 1-5 and the physical properties(FA) of the films manufactured are shown in Table A below.

COMPARATIVE EXAMPLE 1

By way of a comparison, with the film-forming compositions of Examples1-5 a three-layer bioriented film was manufactured by the air bubbletechnique, proceeding as described hereinunder.

The film-forming compositions were coextruded in the form of a tube.

After quenching and solidification, the extruded tube was heated andinflated into a bubble by applying an internal air pressure and thustransforming the narrow, thick-walled tube into a wide, thin-walledtube.

After this stretching step, the tubular film was cooled and collapsedinto a superimposed lay-flat tape and wound onto cylinders.

The air bubble process orients the film by stretching it simultaneouslyboth longitudinally and transversely, thus imparting shrinkability.

The comparison film obtained in the best working conditions known to theinventors had a thickness of 15 microns and will be denoted hereinbelowby the code C-1.

The physical properties (FA) of the film C-1 and the working conditionsof its manufacturing process (PC) are shown in Tables A-F below incomparison with those of films of the present invention.

EXAMPLES 6-9

The film-forming composition of examples 1-5 was used, changing thestretching conditions as shown in Table B which also shows theproperties of the films obtained in this way.

EXAMPLES 10-12

The film-forming composition of examples 1-5 was used, changing thestretching conditions as shown in Table C which also shows theproperties of the films obtained in this way.

More particularly the MD and TD shrinkage values, which may differ onefrom the other within a wide range, are worthy of note.

EXAMPLES 13-14

The film-forming composition of examples 1-5 was used, changing thestretching conditions as shown in Table D which also shows theproperties of the films obtained in this way.

More particularly, the chain configuration for TD orientation wasmodified as follows:

in Example 13, 4 zones of the tenter frame oven have been devoted topreheating, 2 zones to stretching and no zone to relaxation;

in Example 14, 1 zone of the tenter frame oven has been devoted topreheating, 2 zones to stretching and 3 zones to relaxation.

The data in Table D clearly show that, by increasing the number of zonesof relaxation, the TD shrink tension can be reduced without affectingthe high shrinkage values.

EXAMPLES 15-16

The film-forming composition of examples 1-5 was used, changing thestretching conditions as shown in Table E which also shows theproperties of the films obtained in this way.

The data in Table E show that lower shrink tension values can beobtained, especially in TD.

EXAMPLE 17

The film-forming composition of examples 1-5 was used, increasing themanufacturing line speed to reduce the thickness of the film to 10microns and also changing the stretching conditions as shown in Table Fwhich also shows the properties of the films obtained in this waycompared with the film C-1 which is 15 microns thick.

The data in Table F show that, in spite of the smaller thickness, thefilm of the present invention has greater shrinkage values.

EXAMPLES 18-20

The film-forming composition of examples 1-5 was used, changing thestretching conditions as indicated in Table G which also reports theproperties of the thus obtained films. In examples 19 and 20 themanufacturing line speed was also increased to reduce the thickness ofthe films to 12 and 10 microns respectively. The same configuration ofthe tenter frame oven has been employed in these three examples: 2preheating zones, 2 stretching zones and 2 relaxation zones.

The physical properties (FA) of the films and the working conditions oftheir manufacturing process (PC) are shown in Table G.

EXAMPLES 21-23

The following film-forming compositions were used.

Film-forming composition of the core layer 2 (FIG. 1):

100% of an ethylene/octene-1 copolymer which contains 10% octene-1 (NMRdetermination) and has a density of 0.920 g/cm³ and three melting pointsat 106 ±2; 119 ±2 and 122 ±2° C.

Film-forming composition of the skin layers 1 and 3 (FIG. 1):

87.5%, by weight, of a propylene/ethylene copolymer which contains 3.5%ethylene and has a density of of 0.900 g/cm³ and a single melting pointat 135.9 ±2° C.;

12.1%, by weight, of polypropylene having a density of 0.904 g/cm³ and asingle melting point at 160 ±2° C.;

0.4% of slip and anti-block agents.

Note: the abovementioned blend of layers 1 and 3 has one melting pointat 128 ±2° C.

The procedure of Examples 1-5 was used, changing the stretchingconditions as shown in Table H which also shows the properties of thefilms obtained in this way, together with those of the comparison filmC-2 (see comparative example 2 below).

The ratio between layers 1, 2 and 3 was 1:3:1, respectively.

The data of Table H show that the working conditions of themanufacturing process of the invention may change within very wideranges.

COMPARATIVE EXAMPLE 2

By way of a comparison, with the film-forming compositions of Examples21-23, a three-layer bioriented film was manufactured with the airbubble technique, working under the best conditions known to theinventors. The film manufactured in this way had a thickness of 15microns and is denoted hereinunder by the code C-2. The physicalproperties (FA) of the film C-2 and the working conditions of itsmanufacturing process (PC) are shown in Table H in comparison with thoseof the films of Examples 21-23 of the invention.

EXAMPLE 24

The following film-forming compositions have been used:

Film-forming composition of the core layer 2 (FIG. 1):

as indicated for the core layer of examples 21-23;

Film-forming composition of the skin layers 1 and 3 (FIG. 1):

87.5%, by weight, of a propylene-ethylene-butene-1 terpolymer containing94% propylene, 4% ethylene and 2% butene-1, m.p. 130 ±2° C.;

12.1%, by weight, of polypropylene having a density of 0.904 g/cm³ andm.p. 160 ±2° C.;

0.4% of slip and anti-block agents.

The ratio between layers 1, 2 and 3 was 2:11:2 respectively.

The same general process described in examples 1-5 was used, changinghowever the stretching conditions as indicated in Table I which alsoreports the properties of the films thus obtained.

EXAMPLE 25

The following film-forming compositions have been used:

Film-forming composition of the core layer 2 (FIG. 1):

as indicated for the core layer of examples 21-23;

Film-forming composition of the skin layers 1 and 3 (FIG. 1):

87.5%, by weight, of a propylene-ethylene-butene-1 terpolymer containing91% propylene, 2% ethylene and 7% butene-1, m.p. 126 ±2° C.;

12.1%, by weight, of polypropylene having a density of 0.904 g/cm³ andm.p. 160 ±2° C.;

0.4% of slip and anti-block agents.

The ratio between layers 1, 2 and 3 was 2:11:2 respectively.

The same general process described in examples 1-5 was used, changinghowever the stretching conditions as indicated in Table I which alsoreports the properties of the films thus obtained.

EXAMPLE 26

The following 5-ply barrier film was prepared by following the generalprocess of examples 1-5 but using the film forming compositionsindicated below:

Film-forming composition of the core layer 2 (FIG. 2):

80%, by weight, of an ethylene-vinyl alcohol copolymer containing 44%ethylene;

20%, by weight, of a caprolactam/laurolactam copolymer with m.p.128-132° C.;

Film-forming composition of the skin layers 1 and 3 (FIG. 2):

the same blend used for the skin layers 1 and 3 in examples 1-5;

Film-forming composition of the intermediate layers (tie layers) on bothsides of the core layer 4 and 5 (FIG. 2):

maleic anhydride grafted ethylene-butene copolymer containing 6% byweight of butene.

The ratio between the five layers was 2:2:1:2:2.

The line speed was adjusted to get a film 25 microns thick.

The stretching ratios, the stretching conditions and the properties ofthe thus obtained film are reported in Table I.

                                      TABLE A                                     __________________________________________________________________________           Films of the Examples                                                  PC     1       2       3       4    5    C-1                                  __________________________________________________________________________    L.sub.r                                                                              6:1     6:1     6:1     8:1  9.5:1                                                                              5:1                                  T.sub.p (° C.)                                                                115     115     115     110  110  --                                   T.sub.l (° C.)                                                                115      90     110     115  115  115                                  C.sub.r                                                                              6:1     6:1     6:1     5.7:1                                                                              5.8:1                                                                              5:1                                  T.sub.cp (° C.)                                                               115     115     115     127  127  --                                   T.sub.cs (° C.)                                                               115     115-80  115     127  127  115                                  Tcr (° C.)                                                                     80     --       80      85   85  --                                   __________________________________________________________________________    FA     MD  TD  MD  TD  MD  TD  MD TD                                                                              MD TD                                                                              MD  TD                               __________________________________________________________________________    ST kg/cm.sup.2                                                                       17.8                                                                              35.5                                                                              21.9                                                                              39.7                                                                              29.4                                                                              38.0                                                                              37 21                                                                              44 19                                                                              22.4                                                                              27.3                             S% 120 (° C.)                                                                 58  73  64  66  65  67  67 57                                                                              66 56                                                                              60  64                               H      3.6     2.2     1.7     -- --                                                                              -- --                                                                              2.4                                  G      132     139     142     -- --                                                                              -- --                                                                              137                                  M Kg/cm.sup.2                                                                        4279                                                                              5400                                                                              4221                                                                              5048                                                                              4644                                                                              5766                                                                              -- --                                                                              -- --                                                                              3558                                                                              3457                             TS Kg/cm.sup.2                                                                       1015                                                                              946 1356                                                                              1302                                                                              1494                                                                              1337                                                                              -- --                                                                              -- --                                                                              1194                                                                              1114                             E      103 136 112 102 101 117 -- --                                                                              -- --                                                                              100 110                              Average                                                                              14.1    15.2    14.9    15.8 16.0 15.0                                 thickness μ                                                                TTV μ                                                                              ±1   ±1  ±1.1 ±0.9                                                                            ±1.6                                                                             ±3                               PTV sag mm                                                                            25      20      22     --   --    35                                  snaking mm                                                                            15      20      22     --   --    51                                  __________________________________________________________________________

                                      TABLE B                                     __________________________________________________________________________           Films of the Examples                                                  PC     6       7       8       9    C-1                                       __________________________________________________________________________    L.sub.r                                                                              6:1     6:1     6:1     6:1  5:1                                       T.sub.p (° C.)                                                                115     115     115     100  --                                        T.sub.l (° C.)                                                                115     115      90      95  115                                       C.sub.r                                                                              6:1     6:1     6:1     7.3:1                                                                              5:1                                       T.sub.cp (° C.)                                                               115     117     116     130  --                                        T.sub.cs (° C.)                                                               115     115     115     125-95                                                                             115                                       Tcr (° C.)                                                                     80     115-80  115-80   85  --                                        __________________________________________________________________________    FA     MD  TD  MD  TD  MD  TD  MD TD                                                                              MD  TD                                    __________________________________________________________________________    ST Kg/cm.sup.2                                                                       17.7                                                                              35  20.4                                                                              24.4                                                                              13.9                                                                              33.8                                                                              10 37                                                                              22.4                                                                              27.3                                  S% 120 (° C.)                                                                 58  73  62  66  61  68  56 63                                                                              60  64                                    H      2.5     2.2     1.7     -- --                                                                              2.4                                       G      137     138     144     -- --                                                                              137                                       M Kg/cm.sup.2                                                                        5068                                                                              5409                                                                              5056                                                                              5917                                                                              4462                                                                              5816                                                                              -- --                                                                              3558                                                                              3457                                  TS Kg/cm.sup.2                                                                       914 116 1216                                                                              1125                                                                              1160                                                                              1661                                                                              -- --                                                                              1194                                                                              1114                                  E      110 110         125 85  -- --                                                                              100 110                                   Average                                                                              14.8    15.1    15.0    14.9 15.0                                      thickness μ                                                                TTV μ                                                                              ±1  ±1.4 ±0.8 ±0.9                                                                             ±3                                    PTV sag mm                                                                            23      21     --      --    35                                       snaking mm                                                                            20      28     --      --    51                                       __________________________________________________________________________

                  TABLE C                                                         ______________________________________                                               Films of the Examples                                                  PC       10        11        12      C-1                                      ______________________________________                                        L.sub.r  4.5:1     4.5:1     4.5:1   5:1                                      T.sub.p (° C.)                                                                  100       100       100     --                                       T.sub.l (° C.)                                                                   85        95        95     115                                      C.sub.r  6.2:1     6.2:1     5:1     5:1                                      T.sub.cp (° C.)                                                                 134       134       130     --                                       T.sub.cs (° C.)                                                                 115-95    115-95    115-105 115                                      Tcr (° C.)                                                                       95        85        95     --                                       ______________________________________                                        FA       MD     TD     MD   TD   MD   TD   MD   TD                            ______________________________________                                        ST Kg/cm.sup.2                                                                         7.1    6.1    9.4  9.3  14.3 7.5  22.4 27.3                          S% 120 (° C.)                                                                    20     52     20   56   38   52   60   64                           H        6.1       7.1       6.4     2.4                                      G        122       119       119     137                                      M Kg/cm.sup.2                                                                          2847   3539   3117 3575 3255 3675 3558 3457                          TS Kg/cm.sup.2                                                                          475    377    536  381 1117  516 1194 1114                          E         270    176    263  120  165  155  100  110                          Average  16.0      15.3      14.8    15.0                                     thickness μ                                                                TTV μ ±1.1   ±1.4   ±0.8  ±3                                   PTV sag mm                                                                             --        --        --       35                                      snaking mm                                                                             --        --        --       51                                      ______________________________________                                    

                  TABLE D                                                         ______________________________________                                               Films of the Examples                                                  PC       13          14          C-1                                          ______________________________________                                        L.sub.r  6.1         6.1         5:1                                          T.sub.p (° C.)                                                                  115         115         --                                           T.sub.l (° C.)                                                                   90         115         115                                          C.sub.r  6:1         6:1         5:1                                          T.sub.cp (° C.)                                                                 115         117         --                                           T.sub.cs (° C.)                                                                 115-80      115         115                                          Tcr (° C.)    115-85      --                                           ______________________________________                                        FA       MD      TD      MD    TD    MD    TD                                 ______________________________________                                        ST Kg/cm.sup.2                                                                         15.8    54.7    15.0  27.8  22.4  27.3                               S% 120 (° C.)                                                                   58      68      56    68    60    64                                 H        1.7         1.8         2.4                                          G        143         140         137                                          M Kg/cm.sup.2                                                                          6039    4632    4945  5716  3558  3457                               TS Kg/cm.sup.2                                                                         1273    2011    1110  1508  1194  1114                               E        140     63      123   87    100   110                                Average  15.1        14.3        15.0                                         thickness μ                                                                TTV μ ±1.2     +0.8         ±3                                       PTV sag mm                                                                              18          25          35                                          snaking mm                                                                              20          23          51                                          ______________________________________                                    

                  TABLE E                                                         ______________________________________                                               Films of the Examples                                                  PC       15          16          C-1                                          ______________________________________                                        L.sub.r  6.1         6.1         5:1                                          T.sub.p (° C.)                                                                  100         100         --                                           T.sub.l (° C.)                                                                   95          85         115                                          C.sub.r  5:1         5:1         5:1                                          T.sub.cp (° C.)                                                                 134         134         --                                           T.sub.cs (° C.)                                                                 115-105     115-95      115                                          Tcr (° C.)                                                                       95          85         --                                           ______________________________________                                        FA       MD      TD      MD    TD    MD    TD                                 ______________________________________                                        ST Kg/cm.sup.2                                                                         14.7    2.3     13.9  3.5   22.4  27.3                               S% 120 (° C.)                                                                   44      47      36    46    60    64                                 H        10.3         12         2.4                                          G        104          98         137                                          M Kg/cm.sup.2                                                                          3761    4194    3509  3738  3558  3457                               TS Kg/cm.sup.2                                                                         1378    442     1411  419   1194  1114                               E        122     240     117   268   100   110                                Average  14.1        15.6        15.0                                         thickness μ                                                                TTV μ ±0.8     ±1.5      ±3                                       PTV sag mm                                                                             --          --           35                                          snaking mm                                                                             --          --           51                                          ______________________________________                                    

                  TABLE F                                                         ______________________________________                                                  Films of the Examples                                               PC          17                 C-1                                            ______________________________________                                        L.sub.r     6.1                5:1                                            T.sub.p (° C.)                                                                     115                --                                             T.sub.l (° C.)                                                                     115                115                                            C.sub.r     6:1                5:1                                            T.sub.cp (° C.)                                                                    115                --                                             T.sub.cs (° C.)                                                                    115                115                                            Tcr (° C.)                                                                          80                --                                             ______________________________________                                        FA          MD      TD         MD    TD                                       ______________________________________                                        ST Kg/cm.sup.2                                                                            30.5    34.0       22.4  27.3                                     S% 120 (° C.)                                                                      67      69         60    64                                       H           1.2                2.4                                            G            15                137                                            M Kg/cm.sup.2                                                                             4896    5342       3558  3457                                     TS Kg/cm.sup.2                                                                            1503    1287       1194  1114                                     E           86      112        100   110                                      Average     10.1               15.0                                           thickness μ                                                                TTV μ    ±0.8             ±3                                         PTV sag mm  --                  35                                            snaking mm  --                  51                                            ______________________________________                                    

                  TABLE G                                                         ______________________________________                                               Films of the Examples                                                  PC       18          19          20                                           ______________________________________                                        L.sub.r  5.5:1       5.5:1       5.5:1                                        T.sub.p (° C.)                                                                  110         110         110                                          T.sub.l (° C.)                                                                   90          90          90                                          C.sub.r  8.5:1       8.5:1       8.5:1                                        T.sub.cp (° C.)                                                                 130         130         130                                          T.sub.cs (° C.)                                                                 118         118         118                                          Tcr (° C.)                                                                       90          90          90                                          ______________________________________                                        FA       MD      TD      MD    TD    MD    TD                                 ______________________________________                                        ST Kg/cm.sup.2                                                                         --      --      --    --    --    --                                 S% 120 (° C.)                                                                    51      56      51    55    45    49                                H        3.4         3.8         4.2                                          G        128         127         118                                          M Kg/cm.sup.2                                                                          4780    6990    5220  7800  5100  7800                               TS Kg/cm.sup.2                                                                         1103    1290    1180  1420  1240  1190                               E         109     65      100   67    84    70                                Average  15.1        12.3         9.9                                         thickness μ                                                                TTV μ ±1.0     ±0.9     ±0.7                                      PTV sag mm                                                                              23          18          21                                          snaking mm                                                                              20          16          18                                          ______________________________________                                    

                  TABLE H                                                         ______________________________________                                        Films of the Examples                                                         PC      21        22         23      C-2                                      ______________________________________                                        L.sub.r 5:1       7:1        6:1     5:1                                      T.sub.p (° C.)                                                                 100       100        100     --                                       T.sub.l (° C.)                                                                  90        90         90     115                                      C.sub.r 5:1       7:1        8.4:1   5:1                                      T.sub.cp (° C.)                                                                148       148        162     --                                       T.sub.cs (° C.)                                                                130       130        130     115                                      Tcr (° C.)                                                                     110       110        110     --                                       ______________________________________                                        FA      MD     TD     MD   TD    MD   TD   MD   TD                            ______________________________________                                        ST Kg/cm.sup.2                                                                        9.7     20    11.6  32   11.4 20.7 17.4 28.2                          S% 120   32     43     32   41    29   37   43   56                           (° C.)                                                                 H       8.1       9.5        7.3     1.9                                      G       102       122        130     138                                      M Kg/cm.sup.2                                                                         5260   7257   7662 11516 6930 9629 8000 8083                          TS Kg/cm.sup.2                                                                         610    744    868 1126   803 1023  919  993                          E        107    41     84   34    89   47   108  91                           Average 15.5      15.9       16.1    15.0                                     thickness μ                                                                TTV μ                                                                              ±1.2   ±1.4    ±1.5  ±3                                   PTV sag mm                                                                             24        25         23      35                                      snaking mm                                                                             28        25         20      54                                      ______________________________________                                    

                  TABLE I                                                         ______________________________________                                               Films of the Examples                                                  PC       24          25          26                                           ______________________________________                                        L.sub.r  6.5:1       6.5:1       2.4:1                                        T.sub.p (° C.)                                                                  115         115         110                                          T.sub.l (° C.)                                                                  110         100          90                                          C.sub.r  6.5:1       6.5:1       4.5:1                                        T.sub.cp (° C.)                                                                 145         140         118                                          T.sub.cs (° C.)                                                                 120         111         115                                          Tcr (° C.)                                                                      95-80       88-70       110                                          ______________________________________                                        FA       MD      TD      MD    TD    MD    TD                                 ______________________________________                                        ST Kg/cm.sup.2                                                                         --      --      --    --    --    --                                 S% 120 (° C.)                                                                    31      44      35    52    24    56                                H        4.0         1.7         3.4                                          G        123         122         128                                          M Kg/cm.sup.2                                                                          4700    5600    6031  7245  6629  7496                               TS Kg/cm.sup.2                                                                         580     490     927   540   600   850                                E         90      60      87    54   209    92                                Average  15.3        15.8        25.3                                         thickness μ                                                                TTV μ ±0.9     ±1.1     ±1.1                                      PTV sag mm                                                                              18         --           24                                          snaking mm                                                                              19         --           20                                          ______________________________________                                    

What is claimed is:
 1. A multilayer, biaxially oriented, heat shrinkablethermoplastic film comprising:a) a core layer comprisingi) between 50and 100 parts, by weight of the layer, of an ethylene/alpha-olefin C₄-C₁₂ copolymer, and ii) between 0 and 50 parts, by weight of the layer,of a polymer selected from the group consisting of polyolefin, andmodified polyolefin,the layer having more than one melting point whendetected by differential scanning calorimetry according to ASTM D-3418;and b) two outside layers each comprising a polymer selected from groupconsisting of ethylene/alpha-olefin C₄ -C₁₂ copolymer, ethylene/vinylacetate copolymer, ethylene/acrylate copolymer, propylene homopolymer,propylene copolymer, modified polyolefin, and mixtures thereof;whereinthe total thickness variation in said multilayer film is lower than ±10%according to ASTM D 374-88; and wherein the film is made by a tenterframe process.
 2. A multilayer film according to claim 1 wherein thetotal thickness variation is lower than ±5%.
 3. A multilayer filmaccording to claim 1 wherein the film is oriented at a stretching ratioof between 2:1 and 11:1 in the machine direction, and between 3:1 and12:1 in the transverse direction.
 4. A multilayer film according toclaim 1 wherein the polyolefin of the core layer is selected from thegroup consisting of ethylene/vinyl acetate copolymer, ethylene/acrylatecopolymer, propylene homopolymer, and propylene copolymer.
 5. Amultilayer film according to claim 1 wherein the amount of alpha-olefinin the ethylene/alpha-olefin C₄ -C₁₂ copolymer of the core layer is atleast 7% by weight of the copolymer.
 6. A multilayer film according toclaim 1 wherein the alpha-olefin of the core layer is 1-octene.
 7. Amultilayer film according to claim 1 wherein the film is cross-linked.8. A multilayer film according to claim 1 wherein the two outside layerseach comprisea) between 40 and 60%, by weight of the outside layer, of afirst ethylene/ alpha-olefin C₄ -C₁₂ copolymer, b) between about 20 and30%, by weight of the outside layer, of a second ethylene/alpha-olefinC₄ -C₁₂ copolymer, and c) between 20 and 30%, by weight of the outsidelayer, of an ethylene/vinyl acetate copolymer.
 9. A multilayer filmaccording to claim 8, wherein the first ethylene/alpha-olefin C₄ -C₁₂copolymer is an ethylene/octene-1 copolymer.
 10. A multilayer filmaccording to claim 8, wherein the second ethylene/alpha-olefin C₄ -C₁₂copolymer is an ethylene/octene-1 copolymer.
 11. A multilayer filmaccording to claim 10, wherein the second ethylene/octene-1 copolymercontains from 2.5 to 4.5%, by weight of the copolymer, of octene-1. 12.A multilayer film according to claim 1, wherein the two outside layerseach comprise:a) between 75 and 100%, by weight of the outside layer, ofa propylene copolymer, and b) between 0 and 25%, by weight of theoutside layer, of a polypropylene homopolymer.
 13. A multilayer,biaxially oriented, heat shrinkable thermoplastic film comprising:a) acore layer comprising an oxygen barrier material; b) two intermediatelayers each comprising a modified polyolefin; and c) two outside layerseach comprisingi) between 50 and 100 parts, by weight of each layer, ofan ethylene/alpha-olefin C₄ -C₁₂ copolymer, and ii) between 0 and 50parts, by weight of each layer, of a polymer selected from the groupconsisting of polyolefin, and modified polyolefin,each layer having morethan one melting point when detected by differential scanningcalorimetry according to ASTM D-3418;wherein the total thicknessvariation in said multilayer film is lower than ±10% according to ASTM D374-88; and wherein the film is made by a tenter frame process.